The records you keep of expenses should be separated between:
Business expenses, if you are self-employed or are the owner of a business, and expenses incurred in order to generate income, and
Personal expenses, certain of which may be deducted as:
Adjustments to income - deductions from gross income to determine Adjusted Gross Income (AGI)
Itemized deductions, if you can itemize rather than taking the standard deduction, and
Credits, which are a direct reduction of tax.
During the year you may not know, for example, if you will be able to itemize. You will probably want to keep the necessary receipts in case it turns out that you can itemize. If in doubt about whether you will need to keep a receipt, it may be a good practice is to make a notation on the receipt itself as to what it was for, and then keep these in a desk drawer, file folder, envelope, box, bin or wherever else you will be able to find them when you do your taxes.
Proof of Payment
One of the basic concepts involved in substantiating expenses for federal income tax purposes is proof of payment. Whatever the form of payment, the important data you should have are
Payee
Date
Amount
Purpose of payment
Any transaction reference number
Forms of proof of payment include cash receipts, canceled checks, credit or debit card statements, electronic funds transfer confirmations or statements, and payroll deductions. All these forms of payment are accepted as proof of payment, provided they present the information described above. You may need to add specific notations or attach additional explanatory documentation.